MEASURING THE DEFORMATION OF THE SUPER PRESSURE BALLOON USING THE SIMPLIFIED DIGITAL IMAGE CORRELATION METHOD

Abstract

A super-pressure balloon (SPB) is a vehicle which can fly at a constant altitude for an extended period to perform scientific observations at a fraction of the satellite cost. The balloon is always pressurized to keep its volume constant, which suppresses buoyancy fluctuation due to the difference of internal gas temperature between day and night. We have been developing a lightweight, high strength balloon made of thin polyethylene films and diamond-shaped net with high tensile fibers. Although previous research shows that the tensile strength of the net meets the requirements as a strength member of the SP balloon, the net covering the SP balloons are sometimes broken at its inflation test. It is considered to be due to non-uniform expansion which causes stress concentration, however, there is no method to confirm this hypothesis. In this study, we have developed a simplified digital image correlation (SiDIC) method using the intersection detector, which allowed us to track the diamond-shaped weave of the net to measure the balloon deformation during the pressurization process. Digital image correlation (DIC)—an optical method to measure changes in images—usually requires black spots on the specimen for its analysis. However, such method is not suitable to study the shape of SP balloons, as ink spots on the thin film may affect its strength and weight properties. Thus, we developed this SiDIC approach, which overlays the intersection detector to the DIC. This made it possible for the program to track intersection points on the net without using the ink spots. First, this new method was tested using a rubber balloon covered by black patterns and a diamond- shape polymer net. A series of pictures of the balloon as inflated by air at a constant rate were taken and the deformation was measured using the DIC and SiDIC method to compare the results. From the results, the SiDIC almost agree with the result by the DIC method. The error between the DIC results and the real deformation were 87%, and the error between the SiDIC results were 84%. Next, to identify whether the DIC and SiDIC method can be used on the SPB, a rubber balloon covered just with a diamond- shape polymer net was used. In this test, the DIC method could not measure the deformation accurately and had an error of 56% between the real deformation, whereas the SiDIC had an error of 83%. Therefore, the SiDIC method is suitable for monitoring the deformation of the SPB.